This invention relates to an optical data storage medium.
One class of data storage techniques comprises selectively irradiating the surface of a storage medium, the medium being such that, when irradiated at sufficient intensity and duration, its properties are altered in some manner which is readily detectable. In this way data can be written onto and read from the storage medium. The writing irradiation may be electromagnetic or particulate. The advantage of electromagnetic writing radiation is that the writing (recording) operation need not be carried out in vacuum, while that of particulate writing radiation is that for any specified resolution, or data packing density, the depth of focus is much greater than for electromagnetic writing radiation.
When electromagnetic writing radiation is used, one of the difficulties frequently encountered is that the proportion of the incident energy absorbed by the data recording and storage medium may be small, and the energy required to alter detectably its properties may be large. Either or both of these restrictions may increase the energy required to write a detectable data spot on the data storage medium, which can increase the cost of the radiator or slow down the writing speed, or both.
Strongly absorbent surfaces have been prepared by producing a surface which is rough on a microscopic scale and in which the depth of the texture is large compared with its pitch. Such a surface is described by Spiller et al in Applied Optics Vol. 19, No. 17, 1 Sept., 1980 pp 3022-3026. Such structures in the prior art are essentially characterised by the random nature of their surface texture. This random structuring was regarded as valuable since in general a surface with a random. structure or texture will be more absorptive of incident radiation over a range of wavelengths. The methods proposed for the production of such surfaces, such as implantation and etching, are inherently expensive. Furthermore, surfaces which have a random texture, e.g. those made by etching techniques, cannot easily be replicated because etch pits tend to include undercut areas and are often relatively deep.
The inventors of the present invention have now determined that the random surface textures of the prior art are not necessarily optimal for the absorption of a suitable range of wavelengths, e.g. for radiation in the optical and near infra-red wavelength ranges (generally 400-1050 nanometres). An optical data storage medium has therefore been developed which is characterised by its regular surface texture.
More particularly, in one aspect the invention provides an optical data storage medium which comprises an optically sensitive layer at or adjacent to one surface of the medium, said one surface of the medium comprising a plurality of topographical features or structures disposed in a regularly repeating pattern, said pattern being free from undercutting so that the medium can be produced and/or replicated by molding, casting, embossing or similar process against a tool.
In accordance with another aspect of the present invention, there is provided an optical data storage medium having a surface or surface region which is strongly absorbent of radiation falling within a predetermined band of wavelengths, whereby the surface or surface region can be written upon by such radiation, the said surface or surface region including or having adjacent thereto a layer or heat sensitive material which has a textured surface pattern which is a regular pattern, the pitch of which is smaller than the shortest wavelength to which said surface or surface region can respond to be written upon, and the depth (peak-to-trough) of which pattern is less than 1000 nanometers, the textured surface pattern being free from undercutting so as to be suitable for production or replication by moulding, casting, embossing or similar process against a tool, information being stored on said optical data storage medium by irradiating selected areas on the surface of said medium to heat said heat sensitive material and change thereby the local reflectivity of the surface of the medium at said selected areas.
In a further aspect, the present invention provides an optical data storage medium having a surface or surface region which is strongly absorbent of radiation falling within a predetermined band of wavelengths, whereby the surface or surface region can be written upon by such radiation, the said surface or surface region including or having adjacent thereto a layer of heat sensitive material which has a textured surface pattern which is a regular pattern, the pitch of which is smaller than the shortest wavelength to which said surface or surface region can respond to be written upon, the textured surface pattern being free from undercutting so as to be suitable for production or replication by moulding, casting, embossing or similar process against a tool, and the textured surface pattern being overcoated with a thin film of a material which has a high opacity over the predetermined band of wavelengths, information being stored on said optical data storage medium by irradiating selected areas on the surface of said medium to heat said heat sensitive material and change thereby the local reflectivity of the surface of the medium at said selected areas.
In accordance with a fourth aspect of the present invention there is provided an optical data storage medium having a surface or surface region which is strongly absorbent of radiation falling within a predetermined band of wavelengths, whereby the surface or surface region can be written upon by such radiation, the said surface or surface region including or having adjacent thereto a layer of heat sensitive material which has a textured surface pattern which is a regular pattern, the pitch and depth of which are both smaller than 1000 nanometers, the textured surface pattern being free from undercutting so as to be suitable for production or replication by moulding, casting, embossing or similar process against a tool, information being stored on said optical data storage medium by irradiating selected areas on the surface of said medium to heat said heat sensitive material and change thereby the local reflectivity of the surface of the medium at said selected areas.
In accordance with a fifth aspect, the present invention provides an optical data storage medium having a surface or surface region which is strongly absorbent of radiation falling within a predetermined band of wavelengths, whereby the surface or surface region can be written upon by such radiation, the said surface or surface region including or having adjacent thereto a layer of heat sensitive material which has a textured surface pattern which is a regular pattern comprising an arrangement of grooves or protuberances, the pitch of which is smaller than the shortest wavelength to which said surface or surface region can respond to be written upon, and the depth (peak-to-trough) of which pattern is at least 20 nanometers, the textured surface pattern being free from undercutting so as to be suitable for production or replication by moulding, casting, embossing or similar process against a tool, information being stored on said optical data storage medium by irradiating selected areas on the surface of said medium to heat said heat sensitive material and changes the local reflectivity of the surface of the medium at said selected areas.